This invention relates to a crystallizer comprising an elongate housing having two ends, an elongate trough-shaped part located between the ends, and supply means and discharge means adjacent the ends, while in the longitudinal direction of the housing extends a drivable shaft on which and over which at least one stationary box-shaped cooling element is placed which comprises a back wall and a front wall interconnected by a transverse wall and is provided with a slotted recess for placing the cooling element astride on and over the shaft, the front wall and the back wall each being swept by a sweeping element rotating along with the shaft.
Such a crystallizer is known from EP-A-0 031 134. The crystallization of the solution being passed through the crystallizer takes place through cooling by means of the cooling elements. A problem presenting itself here is that crystals which have formed proceed to adhere to the cooling elements and thereby adversely affect optimum crystallization. To prevent crystal caking and consequent encrustation, sweeping elements have been fitted which are to keep the surface of the front and back wall of the cooling elements clean. By having the sweeping elements rotate along with the shaft, which may carry further compartmenting elements, the front and back wall are scraped clean. Although scraping in a pushing manner (hereinafter also referred to as xe2x80x98push-scrapingxe2x80x99) would be preferable, scraping is done in a pulling manner (hereinafter also referred to as xe2x80x98pull-scrapingxe2x80x99) due to the slotted recess provided in the cooling element to enable it to be fitted astride the shaft. If scraping were done in a pushing manner, the scraping edge of the sweeping part, after leaving the wall surface of the cooling element at the recess, would butt against the edge on the other side of the recess, and then, after being bent over to a considerable extent, proceed to scrape in a pulling manner again after all.
The object of the invention is to improve a crystallizer with cooling elements and sweeping elements of the type described in the opening paragraph hereof, such that scraping can be done in a pushing manner.
This is achieved in accordance with the invention if the sweeping element comprises a shaft part which is mounted on the shaft so as to extend substantially radially and rotate along with the shaft and which carries a sweeping part rotatable about the axis of the shaft part, which sweeping part is provided with at least one sweeping arm which extends substantially radially relative to the shaft part and has a free end edge which by springing means can be brought into a push-scraping contact with the adjacent front or back wall of the cooling element, and which, when leaving the respective wall upon reaching the slotted recess, together with an adjacent part of the sweeping element, springs into that recess, the sweeping part being bearing-mounted on the shaft part for rotation such that the sweeping part, as a result of the sweeping arm butting against an edge of the recess, can rotate about the shaft part until a free end edge of a sweeping arm is springingly disposed in a push-scraping contact with the respective wall again. Thus, the butting of the sweeping arm against the edge of the recess, which has heretofore made push-scraping impossible, has been advantageously and surprisingly utilized to accomplish precisely such push-scraping by converting the sweeping arm""s being, as it were, hooked in the recess into a swiveling movement of the sweeping part about the shaft part, so that the wall portion of the cooling element contiguous to the recess can be approached again from above by a sweeping arm and thus can be scraped clean again with a pushing sweeping arm.
If use is made of one sweeping arm for each sweeping element, the couple generated by the forced swiveling of the sweeping part about the shaft part as a result of the further rotation of the sweeping part while the sweeping arm in the recess remains in contact with the edge of that recess and continues to slide along that edge until the free end edge can be pulled onto the wall portion of the cooling element adjacent to the recess, can be accumulated to bring about, upon termination of the forced swiveling, the further rotation of the sweeping part into the push-scraping position. This is possible, for instance, by storing the couple referred to in a spring motor, or tensioning a spring using a cam track. This operation is rendered simpler if the sweeping part carries two diametrically opposite sweeping arms, while, as a result of one sweeping arm lagging in the recess, the other sweeping arm is brought to its push-scraping position. However, according to a further embodiment of the invention, it is preferred that the sweeping element is provided with at least three sweeping arms extending substantially radially relative to the shaft part while enclosing substantially equal angles relative to each other, and is mounted on the shaft such that in each case two adjacent sweeping arms can press simultaneously and scrapingly against the adjacent wall of the cooling element. Upon taking these measures, when the wall surface is being scraped clean, there will be a leading sweeping arm which scrapes in a pushing manner and a trailing sweeping arm which scrapes in a pulling manner, with both sweeping arms being resiliently pressed against the wall surface. Due to the contact of the two sweeping arms with the wall surface, the sweeping part is restrained from rotation about the shaft part. When the leading arm reaches the recess, it will spring into the recess, whereby the restraint from rotation is removed. When subsequently the leading arm at the end of the recess butts against the edge thereof, the sweeping part will swivel about the shaft part until the previously inactive sweeping arm adjacent to the push-scraping sweeping arm comes into contact with the wall surface. The hitherto leading sweeping arm is thereupon pulled onto the cooling element, thereby restoring a situation where two sweeping arms are springingly pressed onto the wall surface and rotation is blocked. Thus, there is again a leading, push-scraping sweeping arm and a trailing, pull-scraping sweeping arm, with the understanding, however, that the initially leading sweeping arm is now the trailing sweeping arm and the initially trailing sweeping arm has been brought into an inactive position, this without involving supplemental swiveling mechanisms with power storage as described in relation to the use of one or two sweeping arms.
Pressing the sweeping arm against the wall surface of the cooling element can be done in various ways. For instance, it may be provided that each sweeping arm, starting from the shaft part, has a radially extending portion which merges adjacent the free end into a portion bent in the sense of rotation of the shaft. Since the sweeping part is moved through the solution to be crystallized, a pressure will be exerted on it, which will be greater on the, as it were, scoop-shaped push-scraping sweeping arm than on the adjacent inactive sweeping arm whose bent portion guides the solution moving along it in a more flowing manner, which results in a press-on force on the push-scraping sweeping arm, which press-on force keeps that sweeping arm in contact with the wall surface of the cooling element.
However, the springing pressing-on of the sweeping arms can also be realized, in a simple yet highly effective manner, if according to a preferred embodiment of the invention the sweeping element is non-slidably mounted on the shaft and the sweeping arms can yield resiliently, while the distance of the axis of the shaft part to an adjacent wall of the cooling element is smaller than the distance of that axis to the imaginary connecting line between the free ends of two adjacently arranged sweeping arms in the condition of the sweeping element when not mounted on the shaft. Thus both the scraping and the tilting effect are ensured.
According to another embodiment of the invention, this can also be realized if the sweeping element is mounted on the shaft to be springingly slidable, such that the sweeping arms are pressed against the adjacent wall of the cooling element. In this case, use can be made of sweeping parts with sweeping arms which spring or which hardly spring, i.e., are made of relatively rigid or hard material.
The latter sweeping arms have the advantage that they have a better scraping action than sweeping arms made of a material generating more friction, for instance rubber. To accomplish such better scraping action in springing sweeping parts as well, it is preferred, in accordance with a further embodiment of the invention, that in that type of sweeping parts each sweeping part is provided with an elastically springing base part which carries at least three relatively rigid and hard sweeping arms.
During scraping, a thrust wave may form in front of the sweeping arm, so that the boundary layer on the wall surface is less disturbed than is desirable to obtain optimum heat transfer. For this reason, it is preferred, in accordance with a further embodiment of the invention, that the free end of each sweeping arm is provided with a beveled surface in leading position as viewed in the direction of rotation, which surface provides for improved removal along the sweeping arm and thereby improves heat transfer between the solution to be crystallized and the cooling element.
The invention further relates to a discrete, loose sweeping element to be used in a crystallizer as intended hereinabove, which sweeping element to that end preferably comprises a shaft part provided at one end with mounting means for the shaft part to be mounted on a shaft so as to extend radially, and carries a freely rotatable sweeping part: which is provided with at least three, preferably four, sweeping arms which extend substantially radially relative to the shaft part. The mounting means may then be so designed that the sweeping element can be mounted on the shaft both rigidly and for springing sliding movement.
Further, the invention also relates to a discrete, loose, sweeping part to be used in a crystallizer or sweeping element as intended hereinabove, which sweeping part to that end preferably comprises a base part of an elastically springing material rotatably mountable on a shaft part, and at least three, preferably four, sweeping arms of a relatively rigid and hard material. It is also possible that the sweeping part comprises a base part rotatably mountable on a shaft part, and at least three, preferably four, sweeping arms each having, starting from the base part, a radially extending portion which merges adjacent the free end into a bent portion.
Referring to exemplary embodiments represented in the drawings, the crystallizer with cooling elements and sweeping elements with sweeping parts according to the invention will presently, though by way of example only, be further discussed. In the drawings: